Move the bless command to a field in Config

[Alexendoo]

The usage in clippy showed that #157 made the API a bit awkward with regards to the bless command and conflict handling modes, this takes it from

let mut config = Config {
    ..
};
config.with_args(&args, /* bless by default */ false);
if let OutputConflictHandling::Error(err) = &mut config.output_conflict_handling {
    *err = "cargo uibless".into();
}

to

let mut config = Config {
    output_conflict_handling: OutputConflictHandling::Error,
    bless_command: Some("cargo uibless".into()),
    ..
};
config.with_args(&args)

The default conflict handling is now only specified in the Config, no more bool in with_args

It's a separate (optional) field because in order for OutputConflictHandling::Bless + --check to work it had to supply a bless command of its own, but there's not really a sensible default suggestion we can make